one-to-one.md

One-to-One Associations

A one-to-one association states that a model may only be associated with one other model. In order for the model to know which other model it is associated with a foreign key must be included in the record.

Waterline uses the concept of a model attribute to indicate that a record should store a reference to another model. Whenever this attribute is found a foreignKey will be built up in the underlying schema to handle the association.

// A user may only have a single pet
var User = Waterline.Collection.extend({

  identity: 'user',
  connection: 'local-postgresql',

  attributes: {
    firstName: 'string',
    lastName: 'string',

    // Add a reference to Pet
    pet: {
      model: 'pet'
    }
  }
});

// A Pet may have multiple users
var Pet = Waterline.Collection.extend({

  identity: 'pet',
  connection: 'local-postgresql',

  attributes: {
    breed: 'string',
    type: 'string',
    name: 'string'
  }
});

In the above example we are associating a Pet with a User. The User may only have one Pet in this case but a Pet is not limited to a single User. Because we have only formed an association on one of the models, a Pet has no restrictions on the number of User models it can belong to. We can change this and associate the Pet with exactly one User and the User with exactly one Pet.

// A user may only have a single pet
var User = Waterline.Collection.extend({

  identity: 'user',
  connection: 'local-postgresql',

  attributes: {
    firstName: 'string',
    lastName: 'string',

    // Add a reference to Pet
    pet: {
      model: 'pet'
    }
  }
});

var Pet = Waterline.Collection.extend({

  identity: 'pet',
  connection: 'local-postgresql',

  attributes: {
    breed: 'string',
    type: 'string',
    name: 'string',

    // Add a reference to User
    user: {
      model: 'user'
    }
  }
});

Now that both models know about each other you can query the association from both sides. To add an association to a model when creating a record you can use the named attribute you set in the model definition.

Pet.create({
  breed: 'labrador',
  type: 'dog',
  name: 'fido',

  // Set the User's Primary Key to associate the Pet with the User.
  user: 123
})
.exec(function(err, pet) {});

This will create a new Pet record with the User foreignKey set. It will allow you to query a Pet and also retrieve their owners but the User side of the association doesn't know about the Pet. To ensure you can query both ways the User record will need to be updated with the new Pet record. You can do this many ways but a simple nested example may look like this:

Pet.create({
  breed: 'labrador',
  type: 'dog',
  name: 'fido',

  // Set the User's Primary Key to associate the Pet with the User.
  user: 123
})
.exec(function(err, pet) {
  if(err) // Handle Error

  User.update(123, { pet: pet.id }).exec(function(err, user) {});

});

Now that the associations are created you can query the records and include the associated data. To do this the populate option is used. This will add a key to each model returned that contains an object with the corresponding record. Because we set the association on both sides above you could use populate on either side.

Pet.find()
.populate('user')
.exec(function(err, pets) {

  // The pets object would look something like the following
  // [{
  //   id: 1,
  //   breed: 'labrador',
  //   type: 'dog',
  //   name: 'fido',
  //   user: {
  //     id: 123,
  //     firstName: 'Foo',
  //     lastName: 'Bar',
  //     pet: 1
  //   }
  // }]

});

One-to-One with Existing Tables

These one-to-one relationships will also work if you're using a legacy database. You'll have to specify a tableName attribute, along with appropriate columnNames for each field attribute.

In this example, PetBiz prefixes all of their tables and fields with pb_. So the Pet model becomes:

var Pet = Waterline.Collection.extend({

  identity: 'pet',
  connection: 'local-postgresql',
  tableName: 'pb_pets'

  attributes: {
    id: {
      type: 'integer',
      primaryKey: true
    },
    breed: {
      type: 'string',
      columnName: 'pb_pet_breed'
    },
    animal: {
      type: 'string',
      columnName: 'pb_pet_species'
    },
    name: {
      type: 'string',
      columnName: 'pb_pet_name'
    },

    // And here we make the association:
    owner: {
      model: 'user'
    }
  }
});

Meanwhile, the User would look something like this:

var User = Waterline.Collection.extend({

  identity: 'user',
  connection: 'local-postgresql',
  tableName: 'pb_user'

  attributes: {
    id: {
      type: 'integer',
      primaryKey: true
    },
    firstName: {
      type: 'string',
      columnName: 'pb_owner_first'
    },
    lastName: {
      type: 'string',
      columnName: 'pb_owner_last'
    },

    // Add a reference to Pet
    pet: {
      model: 'pet'
    }
  }
});

With just these minor changes to the model, the queries described earlier should work the same.